Fetal tissue, and in particular fetal DNA, is routinely used in prenatal diagnosis and other medical procedures which require an accurate assessment of the genome of the fetus. Currently, the fetal tissue is obtained by the use of amniocentesis, chorionic villus sampling (CVS), fetoscopy, or cordocentesis, as described in Thompson and Thompson Genetics in Medicine, 5th Edition, W. B. Saunders Co., Philadelphia, 1991.
In amniocentesis, a sample of amniotic fluid, which contains fetal cells, is transabdominally removed from the mother, with a needle and syringe. Amniocentesis has inherent associated risks. The major risk is induction of miscarriage which is estimated to occur at 1 in 200 amniocenteses. Other risks include maternal infection and physical damage to the fetus. In CVS, fetal trophoblast tissue is aspirated from the villous area of the chorion transcervically or transabdominally. The rate of fetal loss by this method may be as high as 1 in 100. Cordocentesis provides a method of obtaining fetal blood directly from the umbilical cord with ultrasonic guidance. Each of these invasive methods carries risks to both the mother and the fetus.
Although isolation and enrichment of fetal nucleated blood cells from maternal blood has been attempted, the procedure has been very difficult to optimize, primarily due to the rarity of circulating fetal blood cells, and to the biochemical and physiological similarities between fetal blood cells and maternal blood cells.
Accordingly, it would be desirable to have a non-invasive method for isolating and enriching fetal tissue or fetal DNA. It would also be desirable to have a rapid and reliable method of isolating and enriching rare cells from a population of blood cells. Thus, it would be desirable to have a suitable centrifugation medium for the isolation and enrichment of rare cells, including fetal nucleated red blood cells. Surprisingly, the present invention accomplishes these and other related needs.